The invention relates to a magneto-resistance device comprising a stacked structure, said stacked structure comprising a substrate having a surface on which a multilayer structure is provided.
The invention also relates to a magnetic head comprising such a magneto-resistance device.
Magneto-resistance is that phenomenon whereby the electrical resistance measured along a given path within an appropriate material is found to be influenced by the presence of a magnetic field intersecting the material. The phenomenon can thus be exploited to transcribe probed magnetic field fluctuations into accurately measurable to resistance variations within the material, thereby lending itself to application in fine magnetic field sensors and magnetic heads. Such magnetic heads are used for transferring information to and from a magnetic recording medium, such as a magnetic tape or disc.
The magnitude of the magneto-resistance effect along a given path in a particular material is rendered by the formula: ##EQU1## in which R.sub.0 is the electrical resistance measured along the said path in the absence of a magnetic field, and R.sub.F is the smallest electrical resistance measurable along the same path in the presence of a variable magnetic field. The value of MR is usually expressed as a percentage, and is preferably as large as possible so as to yield maximum attainable sensitivity in sensor applications such as those mentioned above.
A magneto-resistance device as hereabove described is elucidated in U.S. Pat. No. 5,134,533, in which the stacked structure consists of a planar non-conducting substrate which is alternately provided with thin layers of ferromagnetic material (preferably Fe, Co or Ni) and thin layers of non-magnetic material (preferably Cr, V or Ti), the former being antiferromagnetically coupled across the latter and having an in-plane easy axis of magnetisation, the total thickness of the multilayer structure thus formed being of the order of a micron. The electrical resistance of such a stacked structure can be measured by applying a known electrical voltage gradient across the plane of the substrate, thereby inducing a measurable lateral flow of electrical current across the multilayer structure, the ratio of the said electrical voltage and electrical current yielding the value of the electrical resistance. The values thus measured, in both the absence and presence of an external magnetic field, can be employed to calculate the magneto-resistance effect using Formula (1) in the previous paragraph. Typical values of the magneto-resistance effect thus attained with the known stacked structure are of the order of 20%.
In experiments leading to the invention, the magnitude of the magneto-resistance effect demonstrated by such a stacked structure was investigated using an alternative measuring geometry, for which the applied electrical voltage gradient was perpendicular to the multilayer structure, together with the induced flow of electrical current. It was hereby observed that the magneto-resistance effect obtained using this alternative (perpendicular) measuring geometry was considerably larger than that obtained with the common (lateral) measuring geometry, by as much as a factor of five. See, for example, the article by Gijs et at. in Phys. Rev. Lett. 70 (1993), pages 3343-3346.
However, an attendant disadvantage of this perpendicular measuring geometry compared to the lateral measuring geometry is that the absolute values of the electrical resistances measured in the former case are drastically lower than those measured in the latter case, as a result of the huge magnitude-difference between the dimensions of the multilayer structure in the perpendicular direction (of the order of a micrometer) and in the lateral directions (of the order of 100 micrometers, or larger). Such tiny resistance values lead to increased electrical power consumption in magneto-resistance devices employing the perpendicular measuring geometry.
A trivial solution to this problem is to simply increase the total thickness of the multilayer structure to macroscopic values approaching the structure's lateral dimensions, thereby simultaneously increasing the perpendicular resistance of the structure. This, however, is an extremely inefficient, expensive and impracticable measure, in view of the lengthy and costly deposition processes and the relatively large quantities of precious metals which it necessitates. It is of little appeal as a viable solution.